Position sensing and/or tracking systems are used to determine a position, orientation and/or shape of an object in three-dimensional space. Position sensing or tracking systems may be employed wherever detection and/or tracking of the position, orientation and/or shape of an object is desired. In position sensing and/or tracking systems the position, orientation and/or shape of an object is continuously determined, thus following or tracking any movement or deformation of the object.
Applications of position sensing or tracking systems include sensing the relative position between a tool and a part to be machined in computer-aided manufacturing (CAM), determining spatial coordinates of points on an object, and tracking and guiding a medical instrument relative to a representation of a body or a body portion in computer-assisted surgery (CAS) or image guided surgery (IGS), for instance. Position sensing or tracking systems typically employ sensors to obtain measurements indicative of a location of an object, e.g., a position and/or orientation of an object in three-dimensional space, or a shape of an object comprising spatial coordinates of distinguished points of an object.
Sensors employed in position sensing or tracking systems may include, for example, sonic or ultrasonic detectors, magnetic detectors, capacitive detectors, radio-frequency and electro-magnetic detectors. The sensors may be cameras that include electro-optical detectors, for instance.
A sensor typically includes, for example, detectors arranged in two-dimensional arrays. In order to determine the location of a particular point in space, multiple sensors, e.g., multiple cameras, or a single sensor in multiple positions and/or orientations, are used. The process of modelling a relationship between sensor measurements, e.g., detector hits, and corresponding spatial locations and/or orientations is called calibration.
Tsai, R. A. “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses”, IEEE JOURNAL OF ROBOTICS AND AUTOMATION, VOL. RA-3, NO. 4, AUGUST 1987 describes various techniques for camera calibration using model parameters for inferring 3D information from sensor array coordinate measurements, e.g., from a camera image. These techniques involve processes of determining the internal camera geometric characteristics and optical characteristics, termed intrinsic parameters, and/or the spatial position and orientation of the camera frame relative to a predetermined world coordinate system, termed extrinsic parameters. Alternatively, camera calibration may be based on model parameters determining a mathematical (i.e., non-physical) mapping of sensor data to 3D information.